Diversity
Boal (2018) identified three factors that affect the probability that a raptor species will use urban areas: (a) prey availability, favouring generalist species as well as specialist species that can find their prey in urban environments; (b) the characteristics of the urban area, providing not only prey but nesting sites; and (c) tolerance of the species to human presence (Steidl and Powell 2006; Carrete and Tella 2011). The 12 species recorded here inhabit this south Chilean city because these three factors are present.
Mak et al. (2021) analysed 112 studies in 13 European countries, which recorded a total of 18 species of raptors in urban environments. Hogg and Nilon (2015) recorded nine species of raptors in business parks in St. Louis, USA (the commonest were Red-tailed Hawk Buteo jamaicensis, American Kestrel Falco sparverius, and Cooper's Hawks Accipiter cooperii). In contrast, some species avoid urban areas (e.g. Long-eared Owl Asio otus, Great Horned Owl Bubo virginianus, Red-shouldered Hawk Buteo lineatus, Dykstra 2018). Rullman and Marzluff (2014) recorded eight species in urban areas in western Washington, USA (Sharp-shinned Hawk Accipiter striatus, B. jamaicensis, A. cooperii, Northern Pygmy-Owl Glaucidium gnoma, Northern Pygmy-Owl Megascops kennicottii, Barred Owl Strix varia, B. virginianus and T. alba). De La Ossa-Lacayo and De La Ossa (2011) recorded five species of diurnal raptors in a city in Colombia (Yellow-headed Caracara Milvago chimachima, Pearl Kite Gampsonyx swanisonii, Snail Kites Rostrhamus sociabilis, Roadside Hawk Buteo magnirostris and Crested Caracara Caracara plancus), representing 20.8% of the species that geographically should exist in the area. The raptors recorded in Valdivia represent 63.1% of the species expected for neighbouring rural areas (eBird 2021)
In Chile, ornithophagous raptors can take advantage of the high abundance of prey in urban areas, both introduced (e.g., House Sparrow Passer domesticus and Rock Pigeon Columba livia) and native species (e.g., Rufous-collared Sparrow Zonotrichia capensis, Austral Thrush Turdus falcklandii, Eared Dove Zenaida auriculata, Black-chinned Siskin Spinus barbatus), documented in Chilean cities (e.g. Figueroa and Lazzoni 2018; Muñoz-Pedreros et al. 2018, 2020). Díaz et al. (2018) recorded potential prey species for raptors specifically in Valdivia (e.g., P. domesticus, C. livia, T. falklandii, Magellanic Tapaculo Scytalopus magellanicus, S. barbatus, White-crested Elaenia Elaenia albiceps). The presence of prey species, combined with the availability of shelter and suitable nesting areas, may explain why raptors are increasingly associated with urban areas (Boal 2018).
Reproduction and feeding
The availability of suitable nesting sites is one of the determining factors for the success of raptor populations in urban areas (e.g., Newton 2010, McPherson et al. 2016; Mannan and Steidl 2018; Kettel et al. 2018). They may even offer better quality sites for some species than do rural habitats (Chace and Walsh 2006; Cooke et al. 2018; Kettel et al. 2018; 2019). Kettel et al. (2018), in a meta-analysis, compared the breeding performance of raptors in urban and rural populations world-wide. They concluded that in general the birds began to breed earlier and had bigger chicks in urban than rural areas, but that some species raised fewer chicks to the fledgling stage in cities; this suggests that cities may be ecological traps for some species. Tella et al. (1996) proposes that reproductive success and survival rates in adults and chicks may be higher in urban areas due to the smaller number of natural predators.
Raptors that specialise in preying on birds (e.g., Peregrine Falcon Falco peregrinus) are more successful even than in rural areas because the density and biomass of prey are significantly higher in cities (Caballero et al. 2016; Kettel et al. 2019); however, raptors that specialise in small mammals (e.g., Eurasian Kestrel Falco tinnunculus) show a negative response (Kettel et al. 2018) to the urban environment. Species specialising in rodents (sensu Muñoz-Pedreros et al. 2019) manage to nest in Valdivia, for example the nocturnal raptors T. alba and S. rufipes; while they nest in the city, their foraging grounds extend to suburban areas. Similar behaviour was documented for F. tinnunculus in the city of Vienna by Sumasgutner et al. (2014), who detected strong dependence on rodents in the outskirts of the city, and on small birds in the centre. Thus, the home ranges of some raptors may extend to outside the city, where they can satisfy the ecological needs that urban areas do not cover (Chace and Walsh 2004).
Nesting site selection varies among urban raptors, but in general they choose tall, bulky trees (Dykstra et al. 2018); some species avoid nesting close to houses (e.g. B. jamaicensis in Stout et al. 1998; F. tinnunculus in Charter et al., 2007) but others do not (e.g. B. lineatus in Dykstra et al. 2000). In Chile, Rivas-Fuenzalida (2015) documented nesting by White-throated Hawk Buteo albigula in a large park in the city of Concepción (southern Chile); they observed adult individuals and one juvenile hunting on the forested hillsides and also among gardens, parks and buildings in the urban area, showing tolerance of human urban activity. They nested in an exotic tree. The nests in Valdivia were recorded in five species of introduced trees and one native species (Table 2).
Urban ecology and governance
Valdivia offers a wide variety of environments other than human settlements (e.g. forest, shrub, grassland, wetland, open spaces). This great environmental heterogeneity may favour the presence of suitable habitats for these species, providing refuge and suitable hunting grounds, as well as a good food supply. The density of tree cover appears to be a promising indicator of biodiversity in urban forests (Mirski 2020). Increasing the numbers of native shrubs and trees could improve the habitat for raptors and their prey, and this should be taken into consideration by those responsible for the environmental governance of cities in southern Chile. Knowledge of the ecology of the faunal assemblages of cities is vital input information, together with other factors like the characteristics of contemporary urbanisation; this information will empower ecologists to understand and intervene in city planning and management (Ramalho and Hobbs 2012; Kettel et al. 2019). Raptors in cities are subject to a range of threats such as persecution (e.g., Smart et al. 2010; Amar et al., 2012) and collisions with man-made structures (Hager 2009); the latter is another issue that should be considered by planners as well as environmental educators and architects, who need to be made aware of urban raptor populations. Urban ecology needs to mature as a holistic, integrated science of urban systems (McPhearson et al. 2016), recognising them as adaptive systems that are complex and spatially heterogeneous. Their future is hard to predict and control completely, but knowledge of urban ecology and the principles of sustainability can and must be included in planning and design (Wuab 2014).